Method for accurate replication of shaped articles using sinterable powders

ABSTRACT

Shaped article replicas which are proportionately accurate replicas of a shaped article and which are equal, greater or smaller in size than said shaped article are made by preparing a swellable polymer cavity mold from said shaped article; immersing said swellable polymer cavity mold in a swelling agent system under conditions which will enlarge said cavity mold to a predetermined extent; forming a powder compact in said casting mold or a replica thereof and heating said powder compact to produced a proportionately accurate solid replica of a predetermined size.

FIELD OF THE INVENTION

This invention relates generally to the fabrication of dimensionallyand/or proportionately accurate replicas of shaped articles, especiallyarticles having complex shapes, using sinterable powders such as ceramicpowder, metal powder, plastic beads or carbon powder.

BACKGROUND OF THE INVENTION

The fabrication of ceramic, metal and other shaped articles by sinteringpowder compacts, whether formed by die pressing, slip casting, orinjection molding, involves a series of certain basic steps. The rawsinterable powder, such as ceramic or metal powder is first mixed with abinder and sometimes a suitable solvent. The binder-powder mixture isthen formed or shaped and any solvent, if present, is removed. In thefinal step the powder compact is heated under conditions which lead toelimination of the binder material and any solvent that may still havebeen present. Continued heating results in the consolidation of theindividual grains of the sinterable powder, such as ceramic, plastic ormetal powder and the creation of a monolithic object which is free ofvoids.

Although processes such as die pressing, slip casting, and injectionmolding are capable of delivering finished objects of great complexityin a bewildering array of materials, they all suffer from one majordrawback. These processes all involve forming compacted powders intocomplex shapes. Powder compacts of these materials invariably havenumerous voids between the individual constituent powder grains of thecompacted powder. Typically such compacted powder objects contain onlyfrom about 40% to 65% solids, by volume, before heating. Voids mountingto 35% to 60% constitute the remainder. When such compacts are heated,the voids are eliminated, and the powder compact undergoes linearshrinkage on the order of 15%-25%.

When an accurate replica of a shaped article is sought in terms ofeither or both size and proportional configuration, it is necessary tooversize the powder compact to an extent sufficient to offset theshrinkage which occurs during the heating of the powder compact, whichis the last step of the procedure.

By way of illustration, if a precise dimensional replica of a shapedarticle is desired when using an injection molding method, the injectionmold die cavities must be enlarged by an amount equal to the amount ofshrinkage that will take place when the molded powder is heated. Thesame type of enlargement must be made to compensate for shrinkage onheating in all powder forming processes.

The dimensional compensation needed for accurate replica of shapedarticles can be provided by hand enlargement of a mold, produced fromthe article to be replicated. However, this solution to the problem isobviously labor intensive and requires great skill to produce aproportionately accurate oversized mold. Since the careful crafting ofthe oversized molds is obviously time consuming and costly, the use ofthis technique is usually limited, either to replicas where the highcost of the finished article is not a deterrent or to the production oflarge numbers of identically shaped articles, as distinguished frommanufacturing situations where only a few relatively low valued replicasare to be made. In addition to being highly labor intensive, theaccuracy of sculpting to provide shrinkage compensation for complexobjects is completely dependent upon the skill of the artisan. Asobserved by Randall German in the book "Powder Injection Molding",p.255, tool design can involve using the ". . . costly trial-and -errorapproach."

The problem of shrinkage resulting from heating can be avoided, in someinstances if it is acceptable for the replicated article to be producedfrom materials that exhibit only a slight amount of shrinkage. Forexample molten metals or glass can be cast in a mold with very littleresulting shrinkage, but there are only a very limited number ofcircumstances where these materials are acceptable for the finalreplicated article. There also have been attempts to develop specialceramics which undergo a phase change and some degree of expansionduring heating to thereby counteract the volume contraction which wouldotherwise result from the elimination of voids. Unfortunately thesematerials have found only limited use, because other essentialproperties of the final product, such as thermal expansion, strength,fracture toughness, etc. are not adequate for most applications.

DESCRIPTION OF THE PRIOR ART

Replicating shaped articles by molding a sinterable powder into theshape of the article to be replicated followed by heating at hightemperature to produce strength and structural integrity is a techniquewhich has been known for centuries. See for example F. H. Norton, "FineCeramics--Technology and Applications" Robert E. Kreiger Publishing Co.,Malaba, Fla., 1987, pp. 101-129; "Slip-casting of Non-clay Materials"Rado, P. Trans. J. Brit. Ceramic Soc., 72 (7), 291-297 (1973) and"Properties of Slip-Cast Transformation-Toughened Beta"--Al₂ O₃ /ZrO₂Composites" D. J. Green and M. G. Metcalf Am. Ceram. Soc. Bull., 63 6!803-820 (1984).

Artists often wish to produce enlarged versions of works of art.Pantographs are commonly used for enlarging two-dimensional works ofart. Very complex three dimensional pantographs for producing enlargedreplicas of three dimensional works have also been developed but theseare strictly mechanical approaches to the creation of enlarged shapedarticles. While these methods may be acceptable for producing molds forthe production of large volume items, such as toys, where precisereplica is not essential, they do not have sufficient accuracy for theproduction of replicas where accuracy is essential such as for theproduction of dental onlays and inlays.

SUMMARY OF THE INVENTION

It has now been found that a variety of different polymers can be madeto swell to a precise predetermined degree and without dissolving, byimmersion in a properly formulated swelling agent system.

Dimensionally accurate replica of shaped articles using any of theconventional sinterable powders in a powder sintering method can beaccomplished by a very simple technique involving the selection of aswellable polymer which can made to increase in size in a reproduciblemanner and to a pre-selected extent, by immersion in an appropriateswelling agent system. The ability to tailor a swelling agent system andpolymer to swell the polymer to essentially the same extent as theshrinkage resulting from the heating of the sinterable powder selectedfor the replica, allows the fabrication of one or even large numbers ofdimensionally accurate replicas of shaped articles, without thenecessity for time consuming and costly hand labor. This ability alsomakes it possible to produce replicas, which are greater or smaller insize than the original to a predetermined extent and yet remainproportionately accurate in relation to the original.

Thus, in accordance with this invention, if it is desired to produce asolid dimensionally accurate replica of a shaped article using asinterable substance, such as a ceramic powder, a cavity mold of saidshaped article is made by encapsulating the shaped article in aswellable polymer. The swellable polymer molding is then immersed in aswelling agent system at a temperature and for a sufficient time periodto swell said swellable polymer cavity mold to a predetermined extent inan essentially uniform manner. The swelling agent system is capable ofswelling the mold to the equivalent extent needed to compensate for theshrinkage of the ceramic powder composition when heated. Powder compactsare then formed in said cavity mold from one or more sinterable powders,such as a ceramic powder. When such an oversized powder compact slip isheated in a selected atmosphere for a predetermined time and at apredetermined temperature, the enlargement of the oversized powdercompact casting will then exactly compensate for the shrinkage thatoccurs on heating, resulting in a dimensionally accurate replica of theoriginal article, with little or no hand labor.

However due to the difficulty of achieving uniformity in a powdercompact produced in flexible polymeric casting mold, it is preferred toutilize an intermediate step in which a casting of a solidifiablecomposition, such as an alginate dental impression composition is madein the flexible casting mold followed by production of a solid castingmold by casting a more rigid material, such as Plaster of Paris, aroundthe solid replica produced in the flexible casting mold.

As described above this invention also lends itself to the production ofproportionately accurate replicas of shaped articles that are eitherlarger or smaller in size than the original shaped article. This caneasily be accomplished by following the procedure described above withonly slight modifications. For example, a proportionately accuratelarger replica of a shaped article can be produced employing the abovedescribed procedure by using a swellable polymer and swelling agentsystem which has the ability to swell the polymer to a specific greaterextent than the degree of shrinkage which will result from the heatingof the sinterable powder selected for the solid replica. Conversely, aproportionately accurate smaller replica of a given size can be producedby selecting a swellable polymer which can be made to swell only to apredetermined extent which is less than the shrinkage which occurs whenthe powder compact is heated. Obviously, the procedure for producingsuch larger or smaller replicas can be repeated a number of times ifnecessary to produce replicas of the desired size.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graphical representation of the swelling ratios of a certainRTV silicone elastomer when immersed in swelling agents comprised ofmixtures of tetraglyme and limonene in various concentrations.

DETAILED DESCRIPTION OF THE INVENTION

I. The sinterable material

This invention is ideally suited to the accurate reproduction of shapedarticles from a variety of different sinterable materials. While themethod of this invention can be used to replicate many different kindsof shaped articles, such as works of art; it is especially suited to theproduction of dental restorations in the form of dental onlays andinlays.

The first step in the practice of this invention is the selection of thesinterable powder to be used for production of the desired solidreplica. This step is followed by a determination of the precise amountof shrinkage which will occur when the selected sinterable material isfired under specific conditions in accordance with a specific heatingschedule. This shrinkage factor is hereinafter referred to as the"coefficient of shrinkage."

The shrinkage that will take place upon heating a powder compact of asinterable substance will be influenced primarily by the followingfactors:

i) the particle size(s) of the sinterable powder

ii) the degree of packing (bulk density) of the sinterable powder

iii) the time, duration and temperature at which the powder compact(molded sinterable powder) is heated and

iv) the chemical composition and constituents of the sinterable powder.

The accuracy of the determination of the coefficient of shrinkage playsa major role in determining the dimensional and proportional accuracy ofthe final solid replica.

Any sinterable powder can be used in the practice of this invention.Among the sinterable powders that are useful are: clay and claycontaining ceramics, alumina, fused silica, beryllia, urania, calcia,magnesia, spinel, calcium fluoride, magnesium fluoride, titaniumnitride, zirconium boride, silicon carbide, titanium carbide and cermetsas well as metal powders, such as gold, silver, bronze, zinc, tungsten,molybdenum, stainless steel, titanium, chromium, silicon as well asmetal alloys and even non-metals such as wood and graphite. Generally,metal powders are preferred. For dental restorations the preferredmaterials are ceramics, especially zirconium oxide or zirconia. Theliterature is replete with disclosures of the wide variety of availablesinterable materials and methods for casting them into cast articles.See for example, P. Rado, "Slip-Casting of Non-clay Materials," BritishCeramic Society, Transactions and Journal, 72(7), 291-297 (1973). Theselection of the sinterable material to be employed for the slip castingof the final product is dictated almost entirely by the physicalproperties and appearance desired in the final product. The fundamentaltechniques employed in the heating of the final slip casting or powdercompact (these terms are used interchangeably herein) date back tobiblical times and are well documented in the literature. The onlyrequirement of the sinterable powder used in the practice of thisinvention is that after production and heating of the powder compact inthe same manner as was employed in determining its coefficient ofshrinkage, the final casting will have the desired physical anddimensional properties.

The amount of shrinkage that will occur upon the heating of any givensinterable powder is preferably determined experimentally, to permitaccurate measurement and comparison of the dimensions of the powdercompact (molded sinterable powder) relative to the dimensions of thefinal solid sintered casting. When establishing the coefficient ofshrinkage for a given sinterable system each of the factors listed abovefor the determination of the coefficient of shrinkage must be notedcarefully so that the result obtained can be reproduced accurately.

In accordance with conventional slip casting procedures the sinterablepowder is mixed with a suitable liquid which can be water or an organicliquid and then packed into a preferably porous mold. After the liquidhas evaporated or migrated into the mold, the powder compact, or slipcasting, is removed and fired to produce the desired solid article. Theamount of liquid employed is a factor that must be taken into accountalong with other factors in determining the coefficient of shrinkage ofthe sinterable composition. Of particular importance is the size of thesinterable powder particles, which establish the bulk density or packingdensity of the powder compact. It is important to avoid the use ofpacking densities which are too low which can result in warpage or otherdimensional distortions when the slip casting is fired.

II. The swellable polymer

Any polymer that is capable of being swelled in an essentially uniformmanner and to a reproducible extent can be used in the practice of thisinvention, provided the polymer is insoluble in the swelling agentsystem selected for swelling the polymer and provided also that afterimmersion in the swelling agent, the swelled polymer retains sufficientstructural integrity to permit its use in the molding proceduresemployed in the practice of this invention. The ratio of the dimensionsof an unswollen polymer to the same polymer after immersion in aspecified swelling agent system at a given temperature for a specificperiod of time, is hereinafter referred to as the "coefficient ofexpansion."

A wide variety of different polymers are capable of use in the practiceof this invention. The most readily available polymers that are capableof being reproducibly swelled, without dissolving, are elastomericpolymers and cross-linked polymers. Among the useful polymers having theproperties desired for accurate dimensional slip-casting are elastomers,as for example: silicone elastomers, polyurethane elastomers, vulcanizednatural latex, polysulfide elastomers and polyether elastomers. Otherexamples of useful swellable polymers are thermoplastic polyurethanes,styrene/butadiene/styrene block copolymers,styrene/ethylene-butylene/styrene block copolymers, polyether/polyamidecopolymers, ionic polymers and a variety of plastic/rubber alloys. Thesepolymeric materials are well known and are readily available from amultitude of commercial suppliers. The preferred swellable polymersuseful according to this invention are elastomers, more preferablysilicone elastomers, notably the RTV silicones.

III. The swelling agent system

Virtually any organic liquid can be used in the practice of thisinvention, provided that it is essentially non-reactive with theswellable polymer selected for use in accordance with the invention andprovided also that it does not dissolve the swellable polymer. Mixturesof various swelling agents are particularly useful due to thepossibility of adjusting the ratio of a plurality of different swellingagents having different swelling effects on the swellable polymer so asto obtain a specific desired ratio between the coefficient of expansionand the coefficient of shrinkage of the sinterable material selected.

If the swellable polymer selected is a hydrophilic polymer, water aswell as aqueous solutions of salts and other substances such as alcoholsand esters, which are miscible in water, can also be used as theswelling agent system employed in the practice of this invention.

The extremely wide range of possible swelling agent systems that arecapable of use in accordance with invention allows the selection of theswelling agent system to be largely a matter of choice. Considerationssuch as cost, flammability and waste disposability may thereforerepresent the most important factors in the selection due to the factthat there are many different swelling agent systems that can be used toachieve the desired coefficient of expansion. Because the temperatureand the duration of the immersion of the selected polymer all have aninfluence of the degree of swelling of the polymer, it is necessary todetermine the coefficient of expansion by experimentation, if accuratedimensional and/or proportional replica is to be achieved. The procedurefor making this determination is quite simple:

i) A cylindrical casting of the polymer selected for use is cut intodiscs of uniform thickness.

ii) The individual discs are then immersed in a representative varietyof different swelling agent candidates maintained at a desiredtemperature.

iii) The specimen discs are periodically removed and measured toestablish the degree of swelling of the individual discs over time tothereby establish the coefficient of expansion for the various swellingagent systems employed in the experiment. The thickness measurementswill establish a time profile of the length of time needed for theselected swelling agent to fully permeate the specimen discs which willprovide an indication of the time required to achieve essentiallymaximum or near maximum expansion of the selected swellable polymer in areproducible manner. Normally this period of time is on the order of 8to 128 hours at room temperature.

An important feature of this invention resides in the use ofcombinations of swelling agents that can used to produce a ratio ofswelling which is different from that of individual pure swellingagents. This is important because it is very unlikely that any singlepure swelling agent will provide a degree of swelling which will be theprecise ratio of swelling needed to compensate for the coefficient ofshrinkage of the selected sinterable composition or to provided a finalsolid product of the desired size. The use of such swelling agentmixtures permits the adjustment of the coefficient of expansion of theselected swellable polymer to very precisely compensate for thecoefficient of shrinkage of the sinterable material selected. The idealswelling agent combinations that allow for such precise adjustment aremixtures of a swelling agent that has little or no ability to cause theselected polymer to swell, such as tetraglyme, and another swellingagent that has a considerable ability to cause the selected polymer toswell, such as limonene. By testing a series of mixtures of suchcombinations and plotting a curve of the results obtained in graphicalform, a very satisfactory curve can be produced which will allow theselection, by interpolation, of a swelling agent mixture for theselected polymer which will very precisely compensate for thecoefficient of shrinkage of the selected sinterable composition. FIG. 1was drawn from data produced in Example 1 below and illustrates such acurve.

FIG. 1 is a graphical representation of the determination of thecoefficient of swelling of an RTV silicone elastomer (Quick PourDuplicating Material, a product of Ceramco, Inc., Burlington, N.J.)determined by the method described above, by immersion in mixtures oftetraglyme and limonene in the various ratios indicated, at atemperature of 23 degrees centigrade for a period of 26 hours. Theseexperiments demonstrate the ease of selecting a swellable polymer andprotocol using various swelling agent systems, which are capable ofproviding coefficients of swelling extending over a range of slightlyabove 1.0 to 1.42.

The following is a listing of pure swelling agents ranked in the orderof their power to cause swelling of a RTV silicone polymer:

    ______________________________________                                        Pure swelling agent                                                                          Thickness Ratio after immersion                                ______________________________________                                        limonene       1.51                                                           turpentine     1.49                                                           mineral spirits                                                                              1.42                                                           dichloromethane                                                                              1.42                                                           toluene        1.42                                                           ethyl acetate  1.41                                                           2-butanone     1.24                                                           acetone        1.10                                                           2-methoxy ether                                                                              1.08                                                           sulfolane      1.04                                                           N,N-dimethylformamide                                                                        1.03                                                           dimethylsulfoxide                                                                            1.03                                                           furfuraldehyde 1.03                                                           triglyme       1.02                                                           tetraglyme     1.01                                                           ______________________________________                                    

Mixtures of the swelling agent materials are especially useful as theymake it possible to achieve a variety of different coefficients ofexpansion, by employing mixtures of different swelling agents with aselected swellable polymer. This makes it possible to achieve a moreaccurate balance between the coefficient of expansion of the polymerselected and the coefficient of shrinkage of the sinterable powdercomposition selected for the replica.

An important factor in the selection of the swelling agent system to beused in accordance with the method is the rate at which swelling agentevaporates from the swelled polymer specimen as this property isdeterminative of the time allowed for producing a mold of the swelledspecimen before swelling agent evaporation begins to cause shrinkage ofthe polymer back to its original size. Swelling agents which evaporaterapidly, such as dichloromethane and toluene are less favored becausetheir rapid rate of evaporation reduces the amount of time available forproducing the slip casting cavity mold.

To ensure a high level of accuracy in the production of a shaped articlereplica it is preferred to make a determination of the coefficient ofexpansion for each individual batch of swellable polymer selected, dueto possible variations in the coefficient of expansion resulting fromdifferences which can occur due to subtle batch to batch variations.

IV. The procedure

Accurate replication of a complex shaped article can be accomplished inaccordance with this invention by following the procedural steps setforth below:

i) Select the sinterable composition that is to be used for casting thedesired solid replica.

ii) Determine the heating schedule and other conditions that are to beused in heating a powder compact of the sinterable composition toproduce the solid replica.

iii) Prepare a test piece of the selected sinterable composition andtake measurements before and after heating in accordance with theheating schedule and conditions to be used in the final step to therebydetermine the coefficient of shrinkage for the sinterable compositionusing the selected heating protocol.

iv) Select a swellable polymer and swelling agent system and conditions,such that the polymer and swelling protocol will result in a coefficientof expansion that essentially compensates for the shrinkage of thesinterable composition selected in step 2 above, when heated. To achievesuch equivalence, the coefficient of expansion for the selected polymershould be equal to 1 divided by the coefficient of shrinkage of thesinterable selected for use in step 1 above. In other words themathematical relationship between these two measures should satisfy thefollowing equation:

    Coefficient of Expansion=1/Coefficient of Shrinkage

This step may require a minor amount of experimentation to empiricallydefine the swelling agent system, temperature and duration of immersionneeded to obtain the desired coefficient of swelling.

If, however it is intended that the final solid replica be larger orsmaller than the original, this can easily be accomplished by selectinga swellable polymer exhibiting a coefficient of expansion having thedesired ratio relative to the coefficient of shrinkage of the selectedsinterable powder composition. Thus, for example a 1.1:1 enlargement canbe achieved by using a swellable polymer having a 1.4375 coefficient ofexpansion in conjunction with a sinterable powder composition having a0.8 coefficient of shrinkage. A ten percent reduction in the size of theoriginal can be accomplished, for example, by employing a swellablepolymer having a coefficient of expansion in conjunction with asinterable powder composition having coefficient of shrinkage equal to0.8. Whether the replica product is smaller or larger than the originalshaped article, the replica product produced by the method of thisinvention remains proportionately accurate in relation to the originalshaped article.

v) Produce a cavity mold of a swellable polymer, using the shapedarticle to be replicated or a model thereof.

vi) Immerse the swellable polymer mold in the selected swelling agentsystem in accordance with the swelling protocol selected in step 4 abovefor a sufficient period of time to ensure essentially completepenetration of the swelling agent system throughout the swellablepolymer cavity mold.

vii) Fill the swellable polymer cavity mold with a castable substance.

vii) After the castable substance has hardened, remove the solid castingfrom the expanded polymer cavity mold and produce a solid casting moldtherefrom by immersing the enlarged solid casting in a liquid ormoldable composition, such as Plaster of Paris, to produce a solidcavity mold.

ix) Remove the solid casting from the enlarged cavity mold.

x) Produce a compact of a sinterable powder using the solid cavity moldprepared in accordance with steps 8 and 9 above.

xi) After drying, remove the slip casting or powder compact from thesolid cavity mold and subject it to heating in accordance with theheating protocol used in establishing the coefficient of shrinkage forthe sinterable composition.

As noted above, it is possible to prepare the slip casting in theswollen polymer cavity mold, thereby eliminating steps 7-9, but thisabbreviated procedure can result in non-uniformity of the slip castingor distortion due to the flexibility of the polymer cavity mold, thusthe full procedure set forth above is preferred.

V. EXAMPLES Example 1 Determination of the Coefficient of Swelling ofthe Swellable Polymer

Cylindrical samples of an addition curable vinyl polydimethylsiloxaneRTV elastomer (Quick Pour Duplicating Material sold by Ceramco Inc.,Burlington, N.J.) were prepared by mixing equal weights of the catalystand silicone base material. The thoroughly mixed blend was poured into acylindrical mold cavity (12.1 mm Diameter by 60 mm Length). The polymercontained in the cylinder was allowed to cure for approximately thirtyminutes and was then removed from the cylinder cavity using a dieplunger and arbor press. 6-8 mm Thickness samples were sliced from thesolidified polymer samples using a razor blade. The finished specimenswere all right cylinders measuring 12.07 mm in diameter and ranging from6 to 8 mm in thickness. The specimens were allowed to cure for a periodof 24 hours before proceeding to the next step.

Mixtures of limonene and tetraglyme swelling agents were preparedgravimetrically from the pure swelling agents and placed in high densitypolyethylene containers with screw closures. The range of swelling agentsamples prepared included pure limonene, pure tetraglyme and mixtures of20%, 40%, 60% and 80% tetraglyme, the remainder being limonene. Twocylindrical samples of polymer were placed in each of the containers andallowed to equilibrate at 23° C.±1° C. for 24 hours.

The samples were removed from the container and blotted dry with atissue. The diameter of each specimen sample was measured with anelectronic caliper by slowly opening the jaws of the caliper until thesample fell free of the caliper by its own weight. This measurement wasrepeated for each of the specimen samples that had been immersed inswelling agent as well as a standard unswollen polymer sample. Thediameters of the two samples immersed in each of the different swellingagents and swelling agent mixtures were averaged and the coefficient ofswelling resulting from immersion in each of the different swellingagent compositions calculated by determining the ratio of the diameterof the swelled specimen to that of the unswollen standard.

The following results for various mixtures of tetraglyme and limonenewere obtained by these procedures:

    ______________________________________                                        Tetraglyme/Limonene Swelling Agent Mixtures                                   % Tetraglyme  % Limonene                                                                              Swelling Ratio                                        ______________________________________                                        0.0           100.0     1.425                                                 20.0          80.0      1.330                                                 40.0          60.0      1.218                                                 60.0          40.0      1.130                                                 80.0          20.0      1.067                                                 100.0         0.0       1.012                                                 ______________________________________                                    

The values obtained for tetraglyme systems were plotted as shown inFIG. 1. FIG. 1 is a graphical representation of the coefficients ofswelling set forth above. This graphical representation of thetetraglyme/limonene system makes it possible to interpolate between themeasured values to obtain a swelling agent system that will provide anydesired coefficient of swelling falling within the overall range. Thus,if for example a swelling system having a 1.35 coefficient of expansionis needed, interpolation based on FIG. 1, shows that a mixture of 84percent limonene and 16 per cent tetraglyme will provide the desiredcoefficient.

Example 2 Determining the Coefficient Of Shrinkage

A slip mixture of zirconium oxide and water was prepared from 16.00 gzirconium oxide powder (TOSOH Zirconia TZ-3YS powder) and 4.00 gdistilled water. After thorough mixing with a plastic spatula, themixture was allowed to stand for 5 minutes to allow air bubbles toescape. Two castings were made in a series of Plaster of Paris cavitymolds of dental onlays of different sizes. The castings were made byrapidly filling the cavity molds with the slip mixture using adisposable plastic pipette. The castings were allowed to stand forapproximately 5 seconds (this allowed the wall thickness of the castpiece to reach a satisfactory level), after which time any excess slipmixture was removed. Castings of rectangular plates were made inrectangular molds measuring 30 mm by 34 mm by 6 mm. In all cases theresultant castings were left in the mold for approximately ten minutesprior to removal. The unfired zirconia castings were placed on aluminainsulation blocks and allowed to air dry for approximately 12 hours.

The unfired zirconia castings, supported on alumina insulation blocks,were placed in a high temperature furnace (Thermolyne High TemperatureFurnace--maximum temperature 1700 C) and fired in an air atmosphere. Theheating schedule was as follows:

1. Heat to 500° C., increasing temperature at a rate of 10° C. perminute with no hold at 500° C.

2. Heat from 500° C. to 1500° C. increasing temperature at a rate of 2°C. per minute with a 2 hour hold at 1500° C.

3. Cool to room temperature at a temperature reduction rate of 4° C. perminute.

After the fired castings had cooled, their length and breadth wascarefully measured and the ratio of the size of the casting to theoriginal model determined. From these measurements it was determinedthat the coefficient of shrinkage of the zirconia ceramic slip mixturewas 0.765 plus or minus 0.004 based on 5 replicas.

Example 3 Preparation of the Cavity Mold

A dental onlay for a molar tooth was selected as the model of the shapedarticle to be replicated. RTV models of the onlay were prepared byplacing an RTV silicone rubber model of the full size onlay on a glassplate and centering it in a plastic ring (22 mm diameter by 10 mm high).An equal weight mixture of the catalyst and base components of theselected silicone elastomer composition (Quick Pour DuplicatingMaterial, a product of Ceramco, Inc. of Burlington, N.J. whose swellingproperties had previously been determined), were mixed and the mixturepoured into the plastic ring to a depth sufficient to cover the onlay by2 mm. The RTV cast was allowed to cure for approximately 10 minutesbefore removal from the glass plate and plastic ring. The onlay was thenremoved from the cast and the cavity model trimmed with a razor blade sothat all walls were approximately 2 mm thick.

Example 4 Swelling the Cavity Model

The cavity models from example 3 were immersed in 0%-100% (w/w), 20%-80%(w/w), 40%-60% (w/w), 60%-40% (w/w), 80%-20% (w/w) 100%-0% (w/w)mixtures of tetraglyme and limonene and allowed to stand for a period of24 hours after which time the now swelled cavity models were removed andblotted dry.

Example 5 Preparing the Enlarged Model Casting

8.5 g of alginate impression material (First Impression-regular set,Den-Mat Corporation, Santa Monica, Calif.) was mixed thoroughly with 25ml of 10 C distilled water and carefully placed in the swollen RTVcavity model using a spatula. Care was taken to avoid entrainment of airbubbles in the impression composition. The cavity mold was filled tooverflowing and then set aside for ten minutes to allow setting toproceed to completion. When the setting was complete the alginateimpression material was wrapped in a moist towel. A shallow circularplastic tray was filled with a well mixed mixture of 100 g of Plaster ofParis and 50 g of distilled water. The alginate casting was coated withthe Plaster of Paris mixture and placed face down in the plaster filledtray. After about ten minutes, the alginate casting was removed from themold cavity. The mold was then removed from the plastic tray and allowedto air dry for several days.

Example 6 Producing The Powder Compact

A slip mixture of zirconium oxide and water was prepared from 16.00 gzirconium oxide powder (TOSOH Zirconia TZ-3YS powder) and 4.00 gdistilled water. After thorough mixing with a plastic spatula, themixture was allowed to stand for 5 minutes to allow air bubbles toescape. The cavity mold prepared in Example 5, was filled rapidly withthe slip mixture using a disposable plastic pipette and allowed to standfor approximately 5 seconds (this allows the wall thickness of the castpiece to reach a satisfactory level), after which time excess slipmixture was removed, using the plastic pipette. The resultant slipcastings were left in the mold for approximately ten minutes after whichtime they were removed from the mold by inverting the mold and rappingit sharply several times. The zirconia cast onlays were placed onalumina insulation blocks and allowed to air dry for approximately 12hours.

Example 7 Heating the Zirconia Casting

The castings produced in Example 6, supported on alumina insulationblocks, were placed in a high temperature furnace (Thermolyne HighTemperature Furnace--maximum temperature 1700 C) and fired in an airatmosphere. The heating schedule was as follows:

1. Heat to 500° C. increasing temperature at a rate of 1° C. per minute,with no hold at 500° C.

2. Heat from 500° C. to 1500° C. increasing temperature at a rate of 2°C. per minute with a 2 hour hold at 1500° C.

3. Cool to room temperature at a temperature reduction rate of 4° C. perminute.

After the fired castings had cooled, their lengths and breadths werecarefully measured and the ratio of the size of the casting to theoriginal model determined.

The results of these measurements are set forth below:

    ______________________________________                                        % Tetraglyme (w/w)                                                                         % Limonene (w/w)                                                                           Degree Enlargement                                  ______________________________________                                        0.0          100.0        1.103                                               20.0         80.0         1.012                                               40.0         60.0         0.918                                               60.0         40.0         0.853                                               80.0         20.0         0.814                                               100.0        0.0          0.774                                               ______________________________________                                    

From these measurements it was determined that:

1. Following the procedure set forth in the Examples, using the RTVcomposition of Example 1. in combination with a swelling agent mixturecomprised of 20.0% tetraglyme and 80.0% limonene will provide an RTVcomposition whose coefficient of swelling will equal the coefficient ofshrinkage of the ceramic zirconia slip composition used in the aboveexamples.

2. By interpolation of a graphical representation of the ratio ofswelling using a series of mixtures of tetraglyme and limonene, thedegree of enlargement possible by following the procedure set forth inthe examples ranges from 77% (using no swelling agent)to an estimated110% (using pure limolene) of the dimensions of the original onlay modelusing the RTV composition of example 1 in conjunction with the ceramiccomposition of example 2.

Once the coefficient of swelling and the coefficient of shrinkage hasbeen determined for a particular batch of swellable polymer andsinterable composition, it is a simple matter to replicate shapedarticles in the manner described above without the need for furthertesting to determine either the coefficient of swelling for theswellable polymer or the coefficient of shrinkage for the slip ceramiccomposition. For example all that would be required to make additionalreplicas of shaped articles using the swellable polymer and the ceramiccomposition employed in the above examples would be to simply immerse aswellable polymer model produced from the polymer described in Example 1in the selected swelling agent and thereafter proceeding in the mannerdescribed in examples 3 to 7 using the ceramic composition of example 6.Obviously, if it is desired to use either or both, a different swellablepolymer or a different ceramic slip composition or if it is desired toalter any of the procedural steps, such as the heating schedule, forexample, then the procedures set forth in the Examples using the newmaterials and/or the different protocol should be repeated.

While the focus of this invention is primarily directed to the accuratereplica of shaped articles, it will be appreciated that this inventionalso permits the production of precise dimensional enlargements as wellas scaled reductions in the size of shaped articles. Enlargements areproduced utilizing swellable polymers whose coefficient of swelling isgreater than the coefficient of shrinkage of the chosen ceramicmaterial. Reductions are possible by producing powder compacts in moldsproduced from unswollen polymers using slip casting compositions havingreproducible coefficients of shrinkage.

What is claimed is:
 1. A method for producing a proportionately accuratereplica of a shaped article that is equal or greater or smaller in sizethan said shaped article, which comprises:a) preparing a cavity mold ofsaid shaped article by encapsulating said shaped article in a swellablepolymer, b) immersing said swellable polymer cavity mold in a swellingagent system at a temperature and for a sufficient period of time toswell said swellable polymer cavity mold to a predetermined extent in anessentially uniform manner, c) forming a sinterable powder compact insaid cavity mold, and d) heating said sinterable powder compact in aselected atmosphere for a predetermined time or times at one or morepredetermined temperatures.
 2. A method for producing a dimensionallyaccurate solid ceramic replica of a shaped article which comprises:a)preparing a cavity mold of a shaped article by encapsulating all or aportion of said shaped article in a swellable polymer, b) immersing saidswellable polymer cavity mold in a swelling agent system at atemperature and for a sufficient period of time to swell said cavitymold to a predetermined extent, c) forming a sinterable powder compactin said cavity mold, d) heating said sinterable powder compact for atime or times and at such temperature or temperatures and in suchatmosphere as will result in an overall shrinkage of said powder compactas will compensate for the expansion of the swellable polymer cavitymold in step b).
 3. A method for producing a proportionately accuratereplica of a shaped article that is equal or greater or smaller in sizethan said shaped article, which comprises:a) preparing a cavity mold ofsaid shaped article by encapsulating said shaped article in a swellablepolymer, b) immersing said swellable polymer cavity mold in a swellingagent system at a temperature and for a sufficient period of time toswell said swellable polymer cavity mold to a predetermined extent in anessentially uniform manner, c) preparing an enlarged casting of saidshaped article by filling said cavity mold with a castable substance, d)forming a solid casting mold by immersing said enlarged casting in aliquid or moldable composition capable of solidifying for a timesufficient for solidification to take place, e) separating said enlargedcasting from said solid casting mold, f) forming a sinterable powdercompact in said solid casting mold, g) heating said sinterable powdercompact in a selected atmosphere for a predetermined time or times atone or more predetermined temperatures.
 4. A method for producing adimensionally accurate replica of a shaped article which is equal insize to said shaped article which comprises:a) preparing a cavity moldof said shaped article by encapsulating said shaped article in aswellable polymer, b) immersing said swellable polymer cavity mold in aswelling agent system at a temperature and for a sufficient period oftime to swell said swellable polymer cavity mold to a predeterminedextent in an essentially uniform manner, c) preparing an enlargedcasting of said shaped article by filling said cavity mold with acastable substance, d) forming a solid casting mold by immersing saidenlarged casting in a liquid or moldable composition capable ofsolidifying for a time sufficient for solidification to take place, e)separating said enlarged casting from said solid casting mold, f)forming a sinterable powder compact in said solid casting mold, g)heating said sinterable powder compact in a selected atmosphere for apredetermined time or times and at such predetermined temperature ortemperatures as will result in an overall shrinkage of said powdercompact as will essentially equal the expansion of said swellablepolymer.
 5. A method according to claim 1 wherein the coefficient ofexpansion of said swellable polymer is equal to the coefficient ofshrinkage of said sinterable powder compact.
 6. A method according toclaim 1 wherein the coefficient of expansion of said swellable polymeris greater than the coefficient of shrinkage of said sinterable powdercompact.
 7. A method according to claim 1 wherein the coefficient ofexpansion of said swellable polymer is less than the coefficient ofshrinkage of said sinterable powder compact.
 8. A method according toclaim 1 wherein said swellable powder cavity mold is immersed in saidswelling agent system until such time as such swelling agent hasessentially fully permeated said swellable powder cavity mold.
 9. Amethod according to claim 3 wherein the coefficient of expansion of saidswellable polymer is equal to the coefficient of shrinkage of saidsinterable powder compact.
 10. A method according to claim 3 wherein thecoefficient of expansion of said swellable polymer is greater than thecoefficient of shrinkage of said sinterable powder compact.
 11. A methodaccording to claim 3 wherein the coefficient of expansion of saidswellable polymer is less than the coefficient of shrinkage of saidsinterable powder compact.
 12. A method according to claim 3 whereinsaid swellable powder cavity mold is immersed in said swelling agentsystem until such time as such swelling agent has essentially fullypermeated said swellable powder cavity mold.
 13. A method according toclaim 3 wherein said castable substance is an alginate dental impressioncomposition.
 14. A method according to claim 3 wherein said liquid ormoldable composition is Plaster of Paris.
 15. A method according toclaim 1 above wherein said sinterable powder is a ceramic powder.
 16. Amethod according to claim 2 above wherein said sinterable powder is aceramic powder.
 17. A method according to claim 3 above wherein saidsinterable powder is a ceramic powder.
 18. A method according to claim 4above wherein said sinterable powder is a ceramic powder.
 19. A methodaccording to claim 2 wherein said shaped article is a dental restorationin the form of an inlay or an onlay.
 20. A method according to claim 4wherein said shaped article is a dental restoration in the form of aninlay or an onlay.
 21. A method according to claim 1 wherein saidswellable polymer is an elastomer.
 22. A method according to claim 3wherein said swellable polymer is an elastomer.
 23. A method accordingto claim 1 wherein said swellable polymer is a silicone elastomer.
 24. Amethod according to claim 3 wherein said swellable polymer is a siliconeelastomer.
 25. A method according to claim 1 wherein said swellablepolymer is an RTV silicone polymer.
 26. A method according to claim 4wherein said swellable polymer is an RTV silicone polymer.
 27. A methodaccording to claim 1 wherein said swelling agent system is an organicliquid which does not react with or dissolve said swellable polymer. 28.A method according to claim 3 wherein said swelling agent system is anorganic liquid which does not react with or dissolve said swellablepolymer.
 29. A method according to claim 1 wherein said swelling agentsystem is a combination of two or more organic liquids.
 30. A methodaccording to claim 3 wherein said swelling agent system is a combinationof two or more organic liquids.
 31. A method according to claim 2wherein said swelling agent system is a mixture of limonene and triglymeor tetraglyme.
 32. A method according to claim 4 wherein said swellingagent system is a mixture of limonene and triglyme or tetraglyme.
 33. Amethod according to claim 2 wherein said sinterable powder compact iszirconia.
 34. A method according to claim 4 wherein said sinterablepowder compact is zirconia.
 35. A method according to claim 1 whereinsaid sinterable powder compact is a metal powder.
 36. A method accordingto claim 3 wherein said sinterable powder compact is a metal powder. 37.A dental onlay or inlay which is a replica produced by the method ofclaim
 2. 38. A dental onlay or inlay, which is a replica, produced bythe method of claim
 4. 39. A replica of a shaped article produced by themethod of claim 3 that is smaller in size than the replicated originalarticle.
 40. A replica of a shaped article produced by the method ofclaim 3 that is larger in size than the replicated original article. 41.A replica of a shaped article produced by the method of claim
 4. 42. Ina method of producing a solid replica of a shaped article by heating asinterable powder casting made in a casting mold produced by replicatingan expanded swellable polymer casting mold the improvement whichcomprises using a swellable polymer and a sinterable powder compacthaving a coefficient of expansion and a coefficient of shrinkage,respectively wherein the mathematical relationship of said coefficientssatisfies the following equation:

    Coefficient of Expansion=1/Coefficient of Shrinkage.


43. In a method of producing a solid replica of a shaped article byheating a ceramic powder compact made in a casting mold produced byreplicating an expandable swellable polymer casting mold the improvementwhich comprises immersing said swellable polymer casting mold in aswelling agent system for a time and at a temperature sufficient toswell said swellable polymer casting mold to a predetermined extentwhich essentially compensates for the shrinkage of said ceramic powdercompact.